In the field of optical communication, there is a pressing need to improve the capacity of optical networks and the speed of switching at reasonable cost. These are attended by the related problems of efficient retrofit to existing infrastructure, ease of maintenance, reliability, etc. The physical media of optical fibers used in current generation optical networks have a tremendous as yet untapped reserve capacity. The reasons for this involve various bottlenecks, chief among them, the slow speed of switches for optical data. To switch optical data, either the data on an optically-modulated signal must be converted to electrical modulation and switched by electrical switches or slow mechanical switches must be used. Even the latter involves the slow conversion of optical modulation into electrical signals for control of the mechanical switches. To compensate for the slowness of the conversion and switching processes, substantial parallelism must be introduced into the design of switches resulting in high cost. In either case, currently, there is no analog to the network switches used in electrical networks, where switching introduces minimal delay in the propagation of network signals.
In addition to the switching process per se, the process of generating optical signals—the modulation itself—is slow because of the rise and fall times of current optical modulators. As a result, symbols are much longer than need be, thereby limiting the bandwidth to a level substantially below the potential of the optical media.
A technique called Wavelength Division Multiplexing (WDM) and a refinement called, Dense Wavelength Division Multiplexing (DWDM) are currently used to increase the capacity of optical media using current modulation technology. WDM or DWDM methods increase the transmission rate by creating parallel information channels, each channel being defined by a different light frequency. Another method, Time Division Multiplexing (TDM) exists in which multiple data sequences are interleaved in time-division fashion on a common medium.
WDM or DWDM methods increase the transmission rate by using parallel information channels. The information in each optical channel is carried by a different light frequency. The light frequencies of the channels are combined together and are inserted into the input of a single optical fiber. The combined light frequencies at the output of the fiber are separated into different parallel channels, one for each specific light frequency. Although DWM and DWDM has the ability increase the capacity of a fiber, the number of channels that may be defined has a practical upper limit because of the limited bandwidth of the fiber (optical properties are attuned to a narrow range of frequencies) and because of the ability of the laser sources to contain their energy in very narrow frequency bands.
In TDM, the bits of several parallel channels at the same light frequency are interleaved in a predetermined periodic order to create a single serial data stream. This method is very effective when using a buffer, which accumulates and compresses the data of several channels into a dense serial data stream of a single channel by reorganizing this data with suitable delays. However the data rate permitted by this method as well as others is still limited by the data rate and duty cycle of the light sources (DFB and DBR lasers) themselves. That is, in direct modulation, the power to the laser is switched on and off. The rate at which this can occur has a physical upper limit due to the relatively long recovery time of the lasers and it produces chromatic dispersions due to broadening of the emitted spectral line of the modulated lasers. This is caused by spontaneous emission, jittering, and shifting of the gain curve of the lasers during the current injection. Where modulation is performed in an indirect manner, the lasers are operated in a Continuous Wave (CW) mode and separate modulators perform the modulation of the beam. The modulators are usually made from interference devices such as Mach-Zender's, directional couplers and active half wave-plates combined with polarizers and analyzers. However, an electro-optical must be activated to modulate the beam; to produce phase shifts and polarization changes. Such changes involve the creation and removal of space charges, which change the density of the charge carriers within these electro-optic materials. The formation rate of the space charges is mainly dependent upon the speed and the magnitude of the applied voltage and can be on the order of sub nanoseconds. The charge removal is usually slower and is mainly dependent upon the relaxation time of these materials (lifetime of charge carriers) and can be relatively long. Accordingly, the width of the pulses and the duty cycle of the modulation are dependent limited by the long off-time of the modulators.
These same rise and fall time limitations impose similar limits on the abilities of switches to direct light along alternative pathways according to routing commands and data. At present, there are two major classes of optical switches. In one class, optical signals are converted to electrical signals, routed conventionally, and optical signals generated anew at the output. As discussed above, the process of conversion is slow and involves many parallel channels making such switches costly as well. This class of switches goes by the identifier OEO, which stands for optical-electrical-optical. A second class of switches goes by the identifier OO, which stands for optical-optical. In these switches, no conversion of optical signals to electrical signals takes place. Instead, the optical energy is routed by means of some sort of light diversion process such as a switchable mirror. In one system, micromechanical actuators or so-called MEMS motors are used to move mirrors in response to electrical routing signals. The speed of such switches is again limited by the need to process electrical signals and the slow response of energy conversion in the MEMS motors. The result is a need for multiple channels to be provided and great expense as well as delay in the speed of the signals along the selectable data routes.
At present, the highest bit rate that can be achieved is about 10G bits per channel, which is limited by the modulation rate of the modulators, the pulse width that they produce, and the switching time of the electronic switches.
As a result of the foregoing limitations of the prior art, there is a need for reliable mechanisms for exploiting the physical potential of fiber optic media in terms of data rate, switching, and cost.